Scroll compressors of small sizes are today used for air conditioning and refrigeration applications, such as the air conditioning system of a vehicle. The attraction of a scroll compressor is it has relatively few moving parts, is highly reliable, offers positive displacement, high efficiencies, and low noise levels. The primary components of a scroll compressor are a stationary scroll and a moving scroll, one of which is rotated by 180.degree. and meshed with the other. The moving scroll is mounted on an eccentric crank so that rotation of the drive produces an orbital motion of the scroll body; however, the scroll does not rotate because it is constrained by a device that ensures the scroll remains in the same angular position during the orbiting motion. As the moving scroll orbits, gas is drawn into and trapped within the two scrolls. The gas moves steadily toward the center of the scrolls and the volume of the gas is reduced as the gas moves toward the center of the scrolls where there is a discharge port through the stationary scroll that allows the gas to discharge. A compressor has a built in swept volume and unswept (clearance) volume; the ratio of the two has a direct affect on performance and efficiency of the compressor. The discharge and bypass ports are a major portion of the unswept volume. The larger the unswept volume, the lower the amount of compression there is before the gas is released; conversely, the smaller the unswept volume, the higher the compression before the gas is released. The gas is drawn in, trapped, reduced in volume, and finally open to the discharge port where it is expelled.
Several methods of capacity control are possible with a scroll compressor including speed variation, suction throttling and internal recirculation. While speed variation is an excellent method of capacity control, it typically uses a frequency inverter to provide a wide range of speeds from a standard ac motor which is not practical for automotive applications. In automotive applications, suction throttling is possible, but is limited by temperature rise due to the increasing pressure ratio. Some screw compressors for refrigeration/gas use an integral slide valve in larger sizes and lift valves in the smaller sizes for off loading. These valves release some of the gas that has been drawn in and trapped just before the internal compression has put energy into it. Thus, the off loading achieved is efficient, as very little energy has gone into the gas returned to the compression suction. The same concept can be built into a scroll compressor. Unfortunately, in the small sized compressors used for automotive applications, the complexity involved makes the compressors relatively expensive, and therefore undesirable. Accordingly, it will be appreciated that it would be highly desirable to have simple device for controlling volume in a scroll compressor for automotive use.
In automotive applications, it is desirable to use a single component for several different vehicle models wherein each model has unique cooling requirements. In using a single component, such as a scroll compressor in an air conditioning system, the compressor output needs to be varied to meet the unique requirements and achieve maximum energy efficiency. In the past, complex valving systems were used with the desired results, but at the cost of increased complexity parts count. Increasing the parts count is undesirable because each part is a potential trouble source. Therefore, it will be appreciated that it would be highly desirable to vary the output of a scroll compressor using a minimum number of components.